1. Field of the Invention
This invention is directed to a discharge excitation type short pulse laser device out of various laser devices, and, more particularly, it is concerned with construction of its electrode.
2. Discussion of the Background
For obtaining the laser oscillation, it is the requisite condition that a spatially uniform discharge be carried out in a laser medium.
However, in those short pulse lasers such as excimer laser, TEA CO.sub.2 laser, and so forth, the discharge tends to form a converged arc, because its operating pressure is as high as several atmospheres. In order to prevent this, it has been a usual practice to effect pre-ionization, in which seeds of electrons are uniformly scattered beforehand in the main discharge region prior to such main discharge.
In the following, explanations will be given as to the conventional techniques in reference to FIGS. 1 to 4 of the accompanying drawing.
FIG. 1 is a cross-sectional view showing a ultra-violet (UV) pre-ionization type excimer laser device as disclosed in, for example, "ELECTRONICS", August issue, page 881 (1983), by Sato et al. In the drawing, a reference numeral 1 designates a high tension power source, a numeral 2 refers to a capacitor, a numeral 3 refers to a high resistor, 4 denotes a winding, 5 represents a capacitor, 6a and 6b refer to pre-ionization pins, 7 a gap, 8 a cathode, 9 an anode, 10 a main discharge region, and 11 a switch.
FIG. 2 is a cross-sectional view of a TEA CO.sub.2 laser device as disclosed in, for example, J. Lachambre et al., IEEE Journal of Quantum Electronics: Vol. QE-9, No. 4, 459p (1973); M. Blanchard et al., Journal of Applied Physics: Vol: 45, No. 3, 1311p (1974); and so forth, which is an improvement for removing various disadvantages in the above-described conventional excimer laser device, detailed explanations of which will be given later. It should, however, be understood that, since the circuit system of the laser device shown in FIG. 2 is made identical with that of FIG. 1, its construction differs somewhat from those laser devices disclosed in these technical literature. In the drawing, a reference numeral 12 designates a dielectric member, a numeral 13 refers to a capacitor, a numeral 14 refers to an auxiliary electrode, a numeral 15 denotes a mesh cathode, and 16 a pre-discharge region.
FIG. 3 is a cross-sectional view showing a construction of the electrode section in the TEA CO.sub.2 laser device shown, for example, in Y, Pan et al., The Review of Scientific Instruments: Vol. 43, No. 4, 662p (1972), which is a modified embodiment of the device shown in FIG. 2 above. In the drawing, a reference numeral 17 designates a Pyrex glass tube, a numeral 18 refers to a lead line, a numeral 19 represents a pre-discharge space, a numeral 20 a power feeding section made of copper, a numeral 21 denotes a supporting table made of a plastic material, and 22 an insulating member.
FIG. 4 is another modified embodiment of the laser device in FIG. 2, in which those component parts same as those in FIGS. 2 and 3 are designated by the same reference numerals.
In the following, the operations of these conventional laser devices will be explained.
In FIG. 1, electric charge supplied from the high tension power source is first accumulated in the capacitor 2. Next, when the switch 11 is brought to its conductive state, the charge accumulated in the capacitor 2 shifts to the capacitor 5 via a current loop starting from the capacitor 2, passing through the switch 11, further passing through the anode 9 and the pre-ionization pins 6b by way of an earth line, and returning to the capacitor 2 by way of the pre-ionization pins 6a, the capacitor 5, and the winding 4. During the shifting of the electric charge, an arc discharge takes place in a very small gap between the pre-ionization pins 6a and 6b, from which arc discharge the ultra-violet rays are generated. With this ultra-violet rays, there takes place photo-ionization in the main discharge region 10 (this photo-ionization will hereinafter be called "ultra-violet ray pre-ionization"), whereby more than 10.sup.4 to 10.sup.6 per cubic centimeter of electrons are uniformly supplied in the space of the main discharge region 10 to suppress growth of local streamers and the arc discharge at the time of the main discharge. On the other hand, even during this period, the shifting of the electric change to the capacitor 5 continues, and the voltage across the cathode 8 and the anode 9 is increasing. And, as soon as this voltage reaches the breakdown voltage, there is obtained a spatially uniform pulse discharge in the main discharge region 10 by the effect of the above-mentioned pre-ionization.
Since the operations of the laser device shown in FIG. 2 is the same as those of the device shown in FIG. 1, the operating mechanism of the pre-ionization will be explained hereinbelow. Prior to the switch 11 being brought to the conductive state, there is substantially no potential difference between the mesh cathode 15 and the auxiliary electrode 14. However, as soon as the switch 11 is brought to its conductive state, and the electric charge starts to move from the capacitor 2 to the capacitor 5, a high electric field is generated across the mesh cathode 15 and the auxiliary electrode 14, whereby the discharge takes place in the pre-discharge space 16 through the dielectric member 12, this discharge process being hereinafter called "aerial pre-discharge". The ultra-violet rays to be generated from this discharge is weaker than that from the arc discharge as shown in FIG. 1 above, and the effect of the ultra-violet rays pre-ionization decreases accordingly. In this conventional embodiment, however, a part of the electrons produced in the pre-discharge space 16 rather passes through the mesh cathode 15 and is directly fed to the neighborhood of the mesh cathode 15 in the space of the main discharge region 10, which is considered to become seed electrons for bringing about spatially uniform main discharge.
FIG. 3 is a modified form of FIG. 2, in which the auxiliary electrode as the lead 18 is disposed in the Pyrex glass tube 17 as the dielectric member held on the plastic supporting table 21, and each lead 18 is joined to the feeding section 20 made of copper to maintain it at the equal electric potential. Moreover, the cathode 8 is in such a construction that it has a plurality of projections in order to enable the aerial pre-ionization to occur in the pre-discharge space 19. The operating mechanism is similar to that shown in FIG. 2 above.
FIG. 4 is a schematic diagram showing the dielectric member 12 and the auxiliary electrode 14 in FIG. 2, which have been replaced by the glass tube 14 and the lead 17. The operating mechanism thereof is similar to that of the embodiment shown in FIG. 2.
By the way, in the conventional embodiment shown in FIGS. 2 to 4, the distance between the cathode 8 or the mesh cathode 15 and the glass tube 17 or the dielectric member 12 (this distance being called hereinafter "thickness of the pre-discharge space") gives influence on the electric power to be made to the pre-discharge space 19 or 16, and the thickness per se determines the volume of the pre-discharge space 16 or 19 with the consequence that it constitutes an important factor to decide the number of electrons per unit area considered in terms of a plane parallel with the above-mentioned cathode.
In the embodiment shown in FIG. 2, it is a usual practice to provide the pre-discharge space 16 of a considerably small thickness in comparison with the distance between the cathode 9 and the mesh cathode 15. Though there has so far been no reported cases, in which the influence by this thickness of the pre-discharge space 16 was quantitatively measured, it is apparent that a tendency to be mentioned hereinbelow is present. That is to say, as the thickness of the above-mentioned pre-discharge space 16 becomes small, the starting voltage of the aerial pre-discharge becomes small, with the consequence that the making power to the above-mentioned space 16 becomes small. Accordingly, when the satisfactory pre-ionization effect is to be obtained, it is necessary that the pre-discharge space 16 be given a certain amount of thickness. However, since it is preferable that the ratio of the power to be consumed for the pre-discharge to the power to be used up for the main discharge be kept to the minimum possible extent from the standpoint of the power efficiency for the laser, the thickness of the above-mentioned pre-discharge space should preferably be kept sufficiently short in comparison with a distance between the anode 9 and the mesh cathode 15 (this distance being called hereinafter "main discharge gap length").
In the similar manner, the embodiment shown in FIGS. 3 and 4 are of such construction that the pre-discharge space 19 is provided between the glass tube 17 and the cathode 8 or the mesh cathode 15.
In the following, explanations will be given in further details as to the pre-ionization mechanism of the conventional laser devices shown in FIGS. 2 to 4.
Unlike the conventional embodiment shown in FIG. 1, these heretofore known laser devices feed the seed electrons to bring about the spatially uniform main discharge only in the vicinity of the cathode, not feeding the same uniformly throughout the space for the main discharge region 10. The effectiveness of this system can be explained as follows. That is to say, as has already been reported in, for example, J. I. Levatter et al., "Journal of Applied Physics: Vol. 51, No. 1, page 210 (1980)", for suppressing the arc discharge, it is only sufficient that local development of streamers be prevented by the effect of the space charge field. Therefore, when the seed electrons are fed in the vicinity of the cathode, these seed electrons are attracted by the cathode 9 to form an electron avalanche 23; however, overlapping of these electron avalanches 23 sooner or later would remove the local gradation of the space charge field to thereby be able to prevent the streamers from speeding.
It follows therefore that large pre-ionization effect is obtained with as large a number of the seed electrons as possible per unit area considered in terms of a plane parallel with the cathode being fed.
The conventional discharge excitation type short pulse laser device of the above-described construction had various points of problem to be mentioned in the following.
The device shown in FIG. 1 is of such a construction that the pre-ionization is effected by the ultra-violet rays from both sides of the principal electrodes 8 and 9. With this construction, however, there is a limit to the depth of penetration of the ultra-violet rays, which makes it difficult to widen the breadth of the main discharge region 10. For example, with the excimer laser, there could only be taken out a laser beam having a rectangular cross-section of 6 to 8 mm.times.20 to 25 mm.
The conventional laser device shown in FIG. 2 provides an improved construction contemplated for solving the above-described points of problem, by which it is possible to widen the breadth of the main discharge region 10 owing to effecting the pre-ionization from the back surface of the mesh cathode 15. As has been mentioned in the foregoing, this type of conventional device has, in its ordinary condition, the mesh cathode 15 provided at a certain distance from the dielectric member 12, which is 3 mm according to the example reported by M. Blanchard et al. in Journal of Applied Physics: Vol. 45, No. 3, page 1311 (1974). However, even this type of the laser device has the following points of problem (a) and (b)
(a) It is evidently advantageous from the standpoint of the pre-ionization effect that as many number of electrons as possible out of those electrons produced in the space between the mesh cathode 15 and the dielectric member 12 be caused to pass through the mesh cathode 15 and be fed to the main discharge region. Therefore, if the thickness of the pre-ionization space 16 is made as thin as possible, i.e., if the volume of the pre-ionization space 16 is made small, the aerial pre-discharge input density increases, and the number of electrons produced per unit area, when considered in terms of the plane parallel to the mesh cathode 15, will increase, and further a ratio of the produced electrons being scattered by their collision against molecules until they reach the mesh cathode 15, or a ratio of the produced electrons being extinguished by their re-combination with ions, will desirably decrease. However, as has already been explained with reference to the conventional laser devices, it is impossible, in the aerial pre-discharge, to reduce thickness of the pre-ionization space 16 and yet to cause the making power to remain unchanged (or increase).
(b) When the pulse oscillation of the laser is effected at a quick repeating speed, the cathode 15 is heated by collision of ions against the cathode, on account of which radiation of this generated heat becomes an important factor. Since the space between the mesh cathode 15 and the dielectric member 12 is narrow and is in a state of substantially no convection being present in it, there takes place only the heat transfer based on a temperature gradient. Accordingly, both mesh cathode 15 and the dielectric member 12 should advantageously be brought as close a distance as possible, which however inevitably raises the problem of reduction in the making power for the above-mentioned aerial pre-discharge.
Also, in the conventional device shown in FIG. 4, there is similar problems. In the exemplary device shown in FIG. 3, the electrons produced by the aerial pre-discharge is in the form which can be readily fed. However, there have been various points of problem such that, since it is virtually difficult to maintain the projections in the cathode 8 and the Pyrex glass tube 17 in an accurately parallel arrangement over the entire longitudinal direction of the cathode 8, and to cause the lead 18 to pass straightforward through the center of the Pyrex glass tube 17, there occur irregularities in the lengthwise direction of the cathode 8, i.e., those places where the aerial pre-discharge takes place easily, and those places where such pre-discharge is difficult to take place, or the structure of the cathode per se is complicated to render its manufacturing difficult.